1. Field of the Invention
This invention relates to automatic exposure control circuits and in particular to a circuit utilizing a photocurrent to generate a timing signal in which a variable exposure interval having a maximum interval is obtained.
2. Description of the Prior Art
Automatic exposure control apparatus for use in cameras have generally functioned to evaluate scene brightness and regulate one or more exposure control parameters such as exposure duration or aperture size in correspondence with the level of scene brightness. Scene brightness evaluation is most often performed with light measuring circuits utilizing one or more photosensitive elements positioned in the camera to receive scene illumination. When utilizing the apparatus to control exposure duration, the system typically integrates the output of a photosensitive circuit. When the integrated output reaches a predetermined level, the exposure is terminated. One circuit, for example, includes an R-C network, the resistor component of which is provided by a photocell, whose resistance is functionally related to scene brightness. Another system uses a photovoltaic cell and operational amplifier having capacitive feedback to provide an integrated output of the cell photocurrent. By activating either of the example integration circuits simultaneously with opening of a shutter, the arrangement generates a voltage at the output of the integration circuit that varies at a rate related to scene brightness. A voltage sensitive trigger circuit is responsive to the output of the integration circuit to terminate exposure when such voltage reaches a predetermined level.
When utilizing such automatic exposure control apparatus in very low light conditions, without the use of an auxiliary lighting unit or tripod, the resulting exposure time may well be beyond the limit of time that the average camera operator is capable of holding a camera steady. Blurred photographs result. To prevent such inadvertent blurred photographs, a limited maximum exposure time is incorporated into some automatic cameras; that is, the exposure duration is permitted to vary from the shortest interval to, for example, an interval of approximately 1/30 sec. as scene brightness decreases. At lower levels of scene brightness, all exposures are accomplished at 1/30 sec.
The most general technique for limiting the exposure duration in such cameras, while permitting the interval to vary over a range of intervals between the minimum exposure interval and an exposure interval that corresponds to the hand-held limit is to employ two timing circuits. A first timing circuit utilizing a photosensitive element and capacitor integrates a current proportional to scene brightness to provide a voltage which reaches the triggering value at a time corresponding to the scene brightness. A second timing circuit uses a fixed resistance and a capacitor to provide a rising voltage that reaches the desired triggering value in a predetermined time. Whichever voltage reaches the trigger value first will terminate the exposure. Thus if scene brightness is normal, the exposure interval provided by the first timing circuit (integrated current proportional to scene brightness) will be shorter than that provided by the second timing circuit (integrated current proportional to the predetermined brightness) and the apparatus will operate to terminate the exposure prior to the maximum exposure interval. If scene brightness is lower than normal, the first timing circuit generates an exposure interval that is longer than the maximum; however, the second timing circuit operates to terminate the exposure at the maximum exposure interval.
The direct comparison of the outputs of the two timing circuits permits a smooth transition between the variable exposure interval and the constant, maximum exposure interval. However, two timing circuits are required, each requiring a capacitor for integrating a current. The capacitor is large, particularly as viewed in integrated circuit technology, and expensive. Thus, it is advantageous from a space and cost standpoint to perform the integration in a single capacitor.